The Yellow-Flanked Fairy Wrasse, Cirrhilabrus ryukyuensis, is a reef-associated species from the Western Pacific, ranging from Indonesia and Malaysia through the Philippines and north to Taiwan and the Ryukyu Islands of Japan. Mature males are identified by a conspicuous triangular yellow to pinkish-yellow patch on the flank just behind the pectoral fin, set against a body of pink, orange, or mauve and a head that shifts from greenish-blue to deep blue-purple. Color intensity rises sharply during courtship and can change with mood, so a settled male in good condition looks noticeably different from a newly arrived specimen. Adults reach roughly 3 inches.

Adults should be housed in an aquarium of at least 55 gallons that has matured and stabilized before the fish is introduced. Layout matters more than raw volume: pair generous open mid-water swimming lanes with reef-style rockwork that provides caves, ledges, crevices, and shaded retreats where the wrasse can rest and take cover. Deep sand is unnecessary, as this species does not bury itself. Moderate to strong varied flow suits it well. A tight-fitting lid, canopy, or fine screen is not optional; this is an active fairy wrasse that jumps readily when startled or when social interactions become animated, and an open-top system is a common cause of loss. Care requirements are moderate, hinging mainly on stable water conditions, secure covering, and frequent feeding.